Dementia is a general term for loss of memory and other mental abilities that are severe enough to interfere with daily life. It is caused by physical changes in the brain. Alzheimer's disease is the most common type of dementia, accounting for 50 to 70 percent of cases. Other causes of dementia include vascular dementia, mild cognitive impairment, mixed dementia, dementia with Lewy bodies, Parkinson's disease, frontotemporal dementia, Creutzfeldt-Jakob disease, normal pressure hydrocephalus, Huntington's disease and Wernicke-Korsakoff syndrome.
Alzheimer's disease (AD) is a neurodegenerative disorder currently affecting approximately 4.5 million individuals in the United States. It is a disorder characterized by the accumulation of plaques and tangles in the brain. The neuropathology typically begins in the entorhinal cortex of the hippocampus. Since this area is critical for memory, early memory impairment is the leading clinical symptom. As the disease spreads to other portions of the brain, other cognitive functions become impaired.
Current treatments for AD include the use of cholinesterase inhibitors and memantine, an N-methyl D-aspartate (NMDA) antagonist. While these drugs are capable of producing drug placebo differences in treated populations, neither class of agents has proven effective in slowing the rate of decline or preventing progression of the disease.
Vascular dementia is widely considered the second most common type of dementia. It develops when impaired blood flow to parts of the brain deprives cells of nutrients and oxygen. The diagnosis may be clearest when symptoms appear soon after a single major stroke blocks a large blood vessel and disrupts the blood supply to a significant portion of the brain. This situation is sometimes called “post-stroke dementia”. There is also a form in which a series of very small strokes, or infarcts, block small blood vessels. Individually, these small strokes do not cause major symptoms, but over time their combined effects become noticeable. This type of dementia used to be called “multi-infarct dementia.”
Mild Cognitive Impairment
In dementing illnesses, the biological changes occur in the brains of patients long before the appearance of symptoms. The concept of mild cognitive impairment was developed in an attempt to recognize Alzheimer's disease (AD) in its very earliest clinical expressions in individual who are destined to develop a progressive dementia later in life. Interventions might then be developed to improve the memory impairment or to delay further deterioration to dementia.
Mixed dementia is a condition in which AD and vascular dementia occur at the same time. Many experts believe mixed dementia occurs more often than was previously realized and that it becomes increasingly common in advanced age. This belief is based on brain autopsies showing up to 45 percent of people with dementia have signs of both Alzheimer's and vascular disease.
The concept of mixed dementia is clinically important because the combination of the two diseases may have a greater impact on the brain than either one by itself.
Dementia with Lewy Bodies
Dementia with Lewy bodies is characterized by abnormal deposits of a protein called alpha-synuclein that forms inside the brain's nerve cells. These deposits are called “Lewy bodies” after the scientist who first described them. Lewy bodies have been liked to several brain disorders, including dementia with Lewy bodies, Parkinson's disease and some cases of Alzheimer's.
Parkinson's disease begins by affecting movement, resulting in tremors and shakiness, stiffness, difficulty with walking and muscle control, lack of facial expressions and impaired speech. Parkinson's is another disease in which Lewy bodies are found in the brain. Many individuals with Parkinson's develop dementia in later stages of the disease.
Frontotemporal dementia (FTD) is a rare disorder that affects the front (frontal lobes) and the sides (temporal lobes) of the brain. Because these regions often (but not always) shrink, brain imaging can be useful in diagnosis. There is no specific abnormality associated with all cases of FTD. In one type called Pick's disease, there are abnormal microscopic deposits called Pick bodies, but these are not always present.
Creutzfeldt-Jakob disease (CJD) is a rare, rapidly fatal disorder affecting about one out of 1 million people per year worldwide. It usually affects individuals over the age of 60. CJD is a type of prion disease. These diseases occur when prion protein, which is present throughout the brain, takes on an abnormal three-dimensional shape. This shape gradually triggers the protein throughout the brain to fold into the same abnormal shape, leading to increasing damage and destruction of brain cells. Recently, “variant Creutzfeldt-Jakob disease” was identified as the human disorder believed to be caused by eating meat from cattle affected by mad cow disease. It tends to occur in much younger individuals, and in some cases, teenagers.
Normal Pressure Hydrocephalus
Normal pressure hydrocephalus (NPH) is another rare disorder in which fluid surrounding the brain and spinal cord is unable to drain normally. The fluid builds up, enlarging the ventricles (fluid-filled chambers) inside the brain. As the chambers expand, they can compress and damage nearby brain tissue. The “normal pressure” refers to the fact that the spinal fluid pressure often, although not always, falls within the normal range on a spinal tap.
Huntington's disease is a fatal brain disorder caused by inherited changes in a single gene. These changes lead to destruction of nerve cells in certain brain regions. Scientists identified the gene in 1993. Anyone with a parent with Huntington's has a 50 percent chance of inheriting the gene, and everyone who inherits it will eventually develop the disorder. In about 1 to 3 percent of cases, no history of the disease can be found in other family members.
Wernicke-Korsakoff syndrome is a two-stage brain disorder caused by a deficiency of thiamine (vitamin B-1). Thiamine helps brain cells produce energy from sugar. When levels of the vitamin are too low, cells are unable to generate enough energy to function properly.
Wernicke encephalopathy is the first, acute phase of the disorder and Korsakoff psychosis is the long-lasting, chronic stage. The most common cause is alcoholism, but the syndrome can also be associated with AIDS, cancers that have spread through the body, very high levels of thyroid hormone and other conditions.